it's a nerve agent. think mustard gas or any used in militaty work and you're along the right lines. pymethrin attacks the nerves. a slow agonising death. the cly equivalent to convulsions and cramps while fully awake. that's why they twitch. i'll be back with more details.

It attacks the nervous system, it leaves them paralysed and shuts down thier respiratory system. Slow and painful death!

The reaction is mainly PH balance. It alters their PH to such an extant that they die.

I think regulations also play a hand. When I was on vacation in Egypt, I got attacked by a load so bought some local fly-spray.

It was awesomely effective! This type of spray however is not allowed in the EU or US!

Fly spray contains chemicals (including many organophosphate compounds) that bind to and permanently block the action of an enzyme called acetylcholinesterase.

Acetyl choline (ACh) is the nerve transmitter substance released by motor neurones (at a site called the neuromuscular junction) to stimulate muscle contraction.

The muscles relax (stops contracting) when the ACh is removed from the neuromuscular junction (NMJ) by the action of acetylcholinesterase.

By inhibiting the cholinesterase the fly can no longer break down ACh in the NMJ and so its muscles lock up in a state of tetany (continuous contraction) making flying and respiration impossible.

The South American indians used a similar approach to kill animals for food using poison-tipped blow-darts. They discovered the natural chemical curare which blocks the sites on the muscles where ACh usually locks on to activate muscle contraction. Because the ACh cannot activate the muscle the individual becomes paralysed and can then easily be trapped and butchered.

The toxin poses no threat to the people who eat the food because it is denatured (detroyed) by cooking and digestion.

Anaesthetists still use the same class of agents today (drugs including tubocurarine, atrocurium, cis-atrocurium and pancuronium) to paralyse patients during surgery.

http://www.answerbag.com/q_view/67042

Do innsects feel pain? I always thought they were just biological robots? Flies have got it coming to them though havn't they? I think its best if you let nature do the work for you. I know where about five spiders hang out ( literally) in the kitchen that see off a good number of flies. The rule is, dont run across the floor causing a daughter to scream and drop all the crockery, and you will be protected.

The foregoing explanations are correct, but a bit pedantic.

In simple terms, the fly dies of asphyxiation. Since it can't breath, it quickly becomes unconscious. Then everything stops.

Flies have a much simpler nervous system than, say, humans; yet they probably can feel pain in a rudimentary way. But why would you want to make them suffer? Why inflict pain unnecessarily? The fly may have been annoying to you, but it was not malevolent. It was not trying to hurt you. It was just being a fly, doing what flies do. Why do you want to take revenge on it?

Fly spray is a relatively humane way to kill the little pests. It is fast, compared with say a chemical that would interfere with digestion and cause the creature to starve. The cruelest death for a fly comes with flypaper, where death usually comes from exhaustion after a long struggle against the glue. With fly spray, unconsciousness comes quickly, so even though the fly may twitch it is not feeling any pain as it chokes to death.

Flies do have a place and a purpose. In fact -- perhaps astonishing to some -- they are part of nature's cleanup crew. Flies and their larva (maggots) devour dead flesh and other waste material; so that dead creatures from cockroaches to horses don't just lie around and putrify. In past years, doctors would put maggots into wounds, so they would eat the dead flesh but leave the living flesh untouched -- thus helping avoid a person's death from gangrene or sepsis.

They are certainly irritating, but that really does not justify wishing them a protracted and painful death. May I say that I hope you will grow past such feelings, which are really not very admirable. Just spray your flies, let them die a quick and relatively painless death, and feel relieved to be rid of the annoyance. But don't wish you could be a torturer.

A big tip dont use fly spray it rarely works, use flash spray or any bacteria spray they instantly drop and it takes them ages to die.